A torque control device includes a functional fluid with which a fluid chamber of a hollow object is filled. The functional fluid is in contact with an adjustment rotor. When a magnetic flux passes through the functional fluid, a viscosity of the fluid is varied. The torque control device controls a torque by variably controlling the viscosity of fluid. The torque control device may be a fluid brake device which controls a brake torque braking an adjustment rotor according to the viscosity of functional fluid, or a liquid clutch device which controls a clutch torque transmitted between a hollow object and an adjustment rotor according to the viscosity of functional fluid.
JP 2013-83244A (US 2013/0074793 A1) describes a torque control device in which a functional fluid contains magnetic particles suspended in a base liquid. A space between a hollow object and a rotation shaft is sealed by a seal structure to seal the functional fluid inside the hollow object. Specifically, the seal structure includes a particle seal, a liquid seal and an intermediate fluid. A seal gap is defined between the rotation shaft and the particle seal to regulate a flow of the magnetic particle. The liquid seal seals a clearance between the rotation shaft and the liquid seal by liquid-tightly contacting the rotation shaft when the rotation shaft slidingly rotates. A middle chamber between the particle seal and the liquid seal is filled with the liquefied intermediate fluid.
In such a seal structure, the seal gap between the rotation shaft and the particle seal can reduce wearing caused by the sliding movement. Under this situation, the particle seal restricts the magnetic particle from leaking out of the hollow object through the seal gap communicating with the fluid chamber. Moreover, in the seal structure, the liquid seal located on the outer side of the particle seal achieves the seal function relative to the liquefied intermediate fluid in the middle chamber between the particle seal and the liquid seal, by liquid-tightly and slidingly contacting with the rotation shaft. As a result, the pressure in the fluid chamber is transmitted to the base liquid which flew into the seal gap from the fluid chamber, and further transmitted to the intermediate fluid in the middle chamber, then, is received by the liquid seal, so the base liquid which receives the pressure can be restricted from leaking out. Thus, lowering in the durability caused by the wearing can be restricted, and variation in the torque adjustment characteristic (brake characteristic) caused by the leakage of functional fluid can be restricted.
Generally, the slidingly moving distance is much larger in the seal structure for the rotation shaft than in a seal structure for a linearly moving shaft. Therefore, the requirement for reducing the wearing becomes severe in the seal structure for the rotation shaft.
However, the wearing may be caused even in a case where the seal gap is formed, if a magnetic particle enters and stays in the seal gap between the rotation shaft and the particle seal, from the fluid chamber, for a long period of time.
If the stagnation of magnetic particle arises, on the surface of the perimeter part of the rotation shaft made of metal, the metal oxide film formed on the surface is subject to wear by the sliding movement. As a result, the metal becomes in the exposed state, and the surface of metal is activated. When the surface of metal is in contact with the magnetic particle and the base liquid under a high temperature state, a sludge may be generated on the surface of metal. The sludge expands the seal gap and makes it easy to leak the magnetic particle.